Stoichiometry issues in pulsed-laser deposition of alloys grown from multicomponent targets

We have examined the degree of congruent transfer in pulsed-laser deposition (PLD) of alloy thin films in phases that are stable over a wide range of compositions. SiGe films were deposited by PLD onto high-purity glassy carbon substrates. We analyzed the average composition of these films using Rutherford backscattering spectrometry (RBS), and results show that the deposited films have a higher relative concentration of Ge than the initial targets. We infer that the noncongruent transfer is due to differential scattering in the plume itself. Additionally, the local composition of the particulates was measured by the use of microprobe analysis, and observations of a sintered target of Si and Ge powders and a solidified target of melted Si and Ge were compared. We found that the sintered target produces particulates with a wide range of compositions, whereas the solidified target produces five times fewer particulates with a tighter distribution of compositions. In contrast with the average composition of the films, the average composition of the particulates is the same as that of the targets. These results are discussed in terms of the microstructure of the targets and the melting process at the surface. The implications of these observations for composition determination by laser ablation are discussed.